The present invention relates to a laser apparatus and method and especially to a solid state laser having a laser diode pump which senses and controls the output wavelength of the laser diode for the absorption peak of the laser medium.
Diode lasers and diode laser arrays exploit changes in temperature as a way to vary the output wavelength with temperature. For example, the output wavelength changes approximately 0.2 nanometers per degree Centigrade for AIGaAs devices. For applications using diode lasers or arrays to excite other lasers, such as Nd:YAG, it is important to match the diode laser output to a spectral absorption feature of the laser being pumped. This is needed in most cases to optimize optical power transfer between the pump diodes and the pumped laser. In other cases, it may be desired to ensure operation at an offpeak spectral feature to provide more spatially uniform optical excitation of the laser medium being pumped or to provide controlled transmission for use of the residual light in other processes, such as frequency mixing.
In the past, the sensing and control of the output wavelength of laser diodes and arrays has been done by measuring the temperature of the heat sink or mounting brackets of the laser diodes. By monitoring this temperature as an indirect measurand, it is assumed that the output wavelength is established. A weakness of this technique is that the temperature of the laser diodes may differ significantly from the sensor mounted in the heat sink or bracket. This may occur in cases where the physical separation is considerable, where the diodes are operating at high average power, or where the cooling geometry is complex. In such cases, the use of heat sink or bracket temperature to characterize the output wavelength will lead to inaccurate measurements of diode temperature. Where diode pumping is used to excite a crystal medium, such as Nd:YAG, errors in diode laser or array temperature measurement can lead to inefficient energy transfer. For example, a 5.degree. Centigrade error will produce a wavelength error of over 1 nanometer as compared to a Nd:YAG absorption linewidth of only 0.7 nanometers. The result of such an error is likely to be a significant reduction in laser efficiency.
Another disadvantage of prior art in laser diode temperature control is the expense and complexity of implementation of temperature control. Accurate sensing requires a precision sensor and readout circuit, with extreme care in design needed to ensure that the sensor is mounted close to the laser diode being sensed and controlled. This is often mechanically difficult and requires extra design engineering and fabrication.
In the past, there have been a great many patents dealing with controlling the output of laser diodes in the telecommunications and recording industries. These prior art laser diode control circuits are used for direct beam stabilization rather than the diode pumping of a solid state laser and results in a more complex circuit. Prior art laser diode controls are usually oriented towards thermoelectrically heated or cooled diodes rather than high temperature diodes. High temperature diodes, in contrast to the thermoelectrically heated/cooled diodes, use only simple heaters connected to a heat sink to provide temperature control. Prior art laser diode controls used to affect output wavelength generally use filters, prisms and the like for wavelength detection, which elements are themselves temperature sensitive and not directly linked to the function or absorption of the pump diode output which produces optimal solid state laser output to be optimized by diode control.
Prior U.S. patents which control laser diodes can been seen in the Hori U.S. Pat. No. 4,821,273, and in the Hori et al. U.S. Pat. No. 5,042,042, both for a wavelength and output power stabilizing apparatus for a semi-conductor laser. These patents use two optical elements with differing spectral characteristics to sense wavelength, changes in the diode laser output, and to provide feedback to control diode temperature and the output wavelength. A control signal is provided to control output power levels. The Hori patents deal with control of the output wavelength of a laser diode and uses optical means to sense wavelength changes. The use of dual channels which must be stable over time to provide long term absolute stability of the diode laser adds complexity and demands that the channels have the same spectral performance over temperature to avoid long term wavelength output drifts. These patents use two distinct interference filters to sense the wavelength changes. The Amano U.S. Pat. No. 5,265,115, is for a solid state laser device having a feedback loop. A detector sensing the output power of a solid state laser provides feedback to increase pump power if output sags, thus stabilizing the output power. This patent deals with a diode laser used to pump a solid state laser but does not control the diode wavelength. Under some conditions, such as where the laser output power is dropping because the diode is too hot and its output spectrum does not match well, the absorption spectrum of the laser medium control scheme may worsen the problem. This patent attempts to invoke solid state laser control by brute force control of input pumping power rather than through control of the pump laser diode wavelength.
The Clark et al. U.S. Pat. No. 4,924,471 is a method for increasing the output power of a laser diode. A cooler is used to reduce the temperature of a GaInP laser diode to increase the output power. It deals with controlling the diode laser temperature but the temperature control is used to optimize power not to stabilize the wavelength. The Winston, Jr. U.S. Pat. No. 5,024,535, is a semi-conductor light source temperature measurement device. A semi-conductor temperature sensor is grown onto the structure of a laser diode and is used to control the diode temperature that hits the output wavelength. The control of the wavelength is indirect because it is entirely thermal and does not provide correction for wavelength changes of a diode as it ages. The Yanagawa U.S. Pat. No. 5,287,367, is an apparatus for controlling a semi-conductor laser in which the diode output spectrum is controlled by thermal control of an element adjacent to the semi-conductor laser and is for control of the wavelength to prevent beam alignment shifts in a CD system as opposed to ensuring that diode output matches solid state laser absorption. The laser wavelength is sensed by using a half mirror or filters, gratings, or a prism and two distinct channels are used to sense wavelength shifts rather than in the present invention which uses a single detector channel and absorbing material with a spectral profile and transmission to which the diode laser is to be logged. The Zelenka et al. U.S. Pat. No. 5,313,482, is a method and circuit arrangement for correcting the light power output of a laser diode. Electrical controls are used in conjunction with a power output detector to stabilize and control the output power level of a laser diode against changes over the diode's life. The Maeda U.S. Pat. No. 5,168,503, is a harmonic generator in which a drive current is modulated by a current modulator and the modulated drive current is supplied to a laser beam generator so that the laser beam emitted by the laser beam generator is varied in wavelength.
In contrast to these prior patents, the present invention is a laser diode/diode array temperature sensing and control system using optical transmission to generate the diagnostic signal. The temperature of a laser diode is sensed by measuring the optical transmission of a material with an absorption peak located within the spectral range of laser emission by using changes in transmission. As the laser temperature varies, the heat input to the laser may be varied to control the output wavelength accurately to remain coincident with the absorption peak.